On Saturday I went on a geology tour of D.C. – Callan Bentley, who teaches geology at Northern Virginia Community College, took a group of science writers around the bottom of the zoo, through a bit of Rock Creek Park, over the Duke Ellington Bridge, and down to an abandoned quarry near Georgetown. It was kind of mind-blowing to add this layer of geological history on top of an area I’ve driven through hundreds, probably thousands of times. We used to be under some pretty tall mountains here, a really long time ago.

Sarah Zielinski at Smithsonian wrote on her blog about finding fossils in building stone – Bentley stopped to point out some fossil crinoids in a block of Indiana limestone on the Duke Ellington Bridge. She used my pictures in the blog post, which is exciting, but it is somewhat ego-deflating to note that the first comment is about how the picture isn’t good enough to make out the fossils. Well, ok then. (I think the actual problem is that they aren’t very exciting fossils. We’re not talking, like, T. rex ribcages here.)

Bentley also stopped at one end of the bridge to point out an interesting variation in color:

Up above, it kinda looks like standard aged building; below, it’s white. (I suspect this looks standard because Indiana limestone is used in a lot of buildings.) Here’s what’s happening. Up above, the Indiana limestone has interacted with sulfur in acid rain to make gypsum, which is calcium sulfate. Gypsum is white, but the stone is dark because the gypsum traps bits of dirt and pollution and crud. The stone below that line doesn’t look that way because it gets cleaned regularly – because that’s where graffiti-writers can reach.